At present, optical wireless communication, which uses infrared ray in data transmission between information terminals in offices or homes, in conformity with the Infrared Data Association (IrDA) standard is widespread. In such optical wireless communication, an optical transmitter-receiver includes a light emitting diode (LED) having a certain directionality as a transmitter and a photodiode (PD) having an appropriate field of view as a receiver.
Two terminals each including such an optical transmitter-receiver are placed a short distance from one another, facing each other. The terminals perform line-of-sight communication by intensity modulation with direct detection (IM/DD). Such directed/line-of-eight optical communication is most advantageous to a portable terminal which requires low power consumption, small size, low weight, and low cost, and therefore is widely used. To date, the communication rate of the directed/line-of-sight optical communication is 4 Mbps, and the transmission range thereof is 1 m. In the future, directed/line-of-sight optical communication will be developed to achieve a communication rate of 100 Mbps and a transmission range of 5 m. Directed/line-of-sight optical communication is increasingly widespread among end users through more and more various applications handling moving pictures and the like.
A LAN (local area network) in which communication is performed by IM/DD using infrared light as a medium has been vigorously developed throughout the world.
FIG. 7 shows various forms of Infrared communication, and corresponds to FIG. 1 in Publication 1 (Joseph M. Kahn et al., Proceedings of the IEEE, pp. 265–298, 1997). FIG. 7 is divided into upper and lower rows (line-of-sight and non-line-of-eight, respectively) depending on whether or not line-of-sight communication is used. FIG. 7 is also divided into columns (directed, hybrid, and non-directed) depending on whether a transmitter-receiver has directionality. In an optical wireless LAN in which a plurality of terminals are wirelessly connected to each access point, light needs to be avoided from being blocked by a barrier or people walking in a network space, for example. Therefore, as shown in the lower right corner of FIG. 7, light is diffused and transmitted in a wide field range, and the light is received by a receiver having a wide field of view. Communication in the form of a non-directed/non-line-of-sight diffuse link is promising. Alternatively, a hybrid system shown in the middle of FIG. 7 is used in which a transmitter uses a directional beam and a receiver has a wide field of view. These systems have merit in the construction of flexible LANs, but require high-cost transmitter-receivers having a high level of power-consumption, or multi-stage transponders. These systems find acceptance in heavily used indoor environments such as offices, hospitals, or schools.
Such existing LAN systems employ their own communication forms and communication protocols which are not compatible with the IrDA standards which are widely used for portable terminals and the like. Even though IrDA terminal users desire to interconnect a plurality of terminals, their IrDA communication functions cannot be used. A whole system must be newly introduced. Recently, Kahn et al. proposed in Publication 1 that a simultaneous link is achieved using space division multiplexing among a plurality of terminals having a directed/line-of-sight communication form shown in the upper left corner of FIG. 7. In this proposal, data transmission among all of the terminals is mediated by an angle-diversity receiver and multi-beam transmitter, which together constitute a so-called optical wireless hub.
FIG. 8 shows two examples of an angle-diversity receiver which is a major component of an optical wireless hub, and corresponds to FIG. 22 of Publication 1. In either example shown in FIG. 8, any angle at which signal light comes from corresponds to the coordinate of the position of one of a plurality of photodetectors.
Of the examples shown in FIG. 8, an example in which an imaging lens having a relatively high spatial resolution is used will be particularly described, with reference to FIG. 8(b), which is a diagram showing a configuration of an imaging receiver and FIG. 8(d), which is a diagram schematically showing the spatial resolution of the imaging receiver of FIG. 8(b). In this case, the imaging lens is designed so that an optical signal from any direction is converged to a signal focusing plane. Therefore, an optical signal incident to the imaging lens at a certain angle is detected by a certain cell (and/or cells in the vicinity of the cell) of a monolithic photodetector array which outputs a signal in response to the incident optical signal. The detected signal is amplified by a preamplifier array subsequent to each cell. Of such detected signals, a signal having the highest intensity is selectively processed so that signal sources having different angles with respect to the imaging receiver can be separately identified. In principle, an N-to-N simultaneous communication is possible.
However, there are various problems to be overcome in order that the portable terminals are directly incorporated into a high-speed LAN having a random multiple access capability. One of the problems is that transmission and reception cannot be simultaneously conducted in the communication between the portable terminals in conformity with the IrDA standard, limiting the communication to a half-duplex communication. The major physical factor of such a problem is that transceivers must be simple, small, and inexpensive and therefore the transceivers cannot have a structure for preventing transmitted light from diffracting and returning to the transceivers that have transmitted the light (e.g., a receiver and a transmitter are positioned at a sufficient distance from each other).
Further, in conventional optical wireless LANs, optical transmission and reception may be conducted using a signal optical channel (e.g., diffuse light in a single wavelength band covers an entire network area). Such communication is limited to one-way 1-to-N (broadcast) communication. Time division multiplexing (TDM) is introduced to the communication, thereby making it possible to conduct time division multiplex access (TDMA). When a system interconnects a plurality of terminals, it is difficult to significantly increase a transmission rate between each terminal and the power consumption of the whole system is increased. The system may interconnect a plurality of terminals using a so-called cellular communication system in which a network space is divided into space cells using a plurality of beams having a certain level of directionality. In this case, when TDMA is conducted in half-duplex communication, it must be verified that other terminals have already conducted a communication, just before each terminal starts communicating on the LAN. Such a verification procedure is called collision avoidance. Even when the collision avoidance procedure is conducted, if a terminal in a bad communication state (hidden terminal) exists within an area, a communication error may occur.
Even when a code is assigned to each communication channel (CDMA) or a carrier frequency is assigned to each communication channel (FDMA), i.e., multiplexing using an electric circuit, a communication capacity per user is limited. In this case, signal processing is very complex, and the power consumption of the whole system is inevitably increased. Even when CDMA or FDMA is combined with the cellular communication system in the LAN, simultaneous communication among a plurality of terminals causes interference among the signals. Therefore, the conventionally well-known collision detection procedure is indispensable. Therefore, a waiting time and an extra signal processing are required for each terminal, so that it is difficult to provide a satisfactory high-speed LAN environment.
However, in wavelength division multiple access (WDMA) in which a communication wavelength is assigned to each channel, multiple access can, in principle, be simultaneously conducted in a diffuse link. In this case, the wavelength of a light source of each transmitter needs to be variable. Conversely, when a light source of each transmitter has a constant wavelength and a plurality of wavelength bands are used, a receiver requires a bandpass filter in which only single wavelengths are selected from all of the wavelength bands used in a link and a center wavelength of transmission is variable. Such functions are not easily achieved in a single device at low cost. Accordingly, a transmitter including a plurality of light sources each having a constant wavelength and a receiver including a plurality of filters each having constant bandpass characteristics are required for each terminal, so that a practical system is not achieved.
The object of the present invention is to provide a high-speed and large-capacity LAN in which a plurality of terminals can be simultaneously interconnected without producing significant load on the terminals and in which improved communication abilities (longer distance or higher speed) of the terminals are directly achieved, using the merits of the directed/line-of-sight optical communication which is widespread for use in portable terminals.